Problem 58 - Chemistry and Chemical Reactivity

Question

Identify the following as either \(p\) - or \(n\) -type semiconductors. (a) germanium doped with arsenic (b) silicon doped with phosphorus (c) germanium doped with indium (d) germanium doped with antimony

Step-by-Step Solution

Answer

(a) n-type, (b) n-type, (c) p-type, (d) n-type.

Step 1: Understand Semiconductor Doping

Semiconductors like germanium and silicon can be doped with different elements to either add extra electrons or create "holes" where electrons are missing. An element that donates electrons creates an n-type (negative) semiconductor, while an element that creates holes results in a p-type (positive) semiconductor.

Step 2: Germanium doped with Arsenic

Arsenic (As) has five valence electrons, which is one more than germanium's four valence electrons. When germanium is doped with arsenic, the extra electron from arsenic becomes a free electron. This makes germanium doped with arsenic an n-type semiconductor.

Step 3: Silicon doped with Phosphorus

Phosphorus (P) also has five valence electrons, one more than silicon's four valence electrons. When phosphorus is used to dope silicon, it provides an extra electron that contributes to electrical conductivity, making it an n-type semiconductor.

Step 4: Germanium doped with Indium

Indium (In) has three valence electrons, which is one less than the four needed to form stable bonds with germanium. This creates a hole, where an electron is missing, leading to increased hole concentration. Therefore, germanium doped with indium becomes a p-type semiconductor.

Step 5: Germanium doped with Antimony

Antimony (Sb) has five valence electrons, similar to arsenic. When it dopes germanium, the extra electron becomes free, enhancing conductivity. Thus, germanium doped with antimony is an n-type semiconductor.

Key Concepts

Doping

Doping is a critical process in modifying the electrical properties of semiconductors, which are materials that have a conductivity between conductors and insulators. It involves intentionally introducing impurities into semiconductor materials like germanium and silicon to improve their conductivity. This is crucial because pure semiconductors do not have ample charge carriers to conduct electricity efficiently.
When a semiconductor is doped, there are two possible outcomes depending on the type of impurity added:

• If the impurity adds extra electrons, it creates an n-type semiconductor.
• If the impurity creates holes by taking away electrons, a p-type semiconductor is formed.

The choice of impurity significantly impacts the semiconductor's behavior, allowing for the creation of electronic components like diodes and transistors that are essential in modern technology.

N-type Semiconductor

An n-type semiconductor is created when the semiconductor material is doped with elements that have more valence electrons than the host material. These elements are usually from group V of the periodic table, like phosphorus, arsenic, and antimony.
During the doping process, the extra electrons from these impurity atoms become free to move, thereby increasing the material's conductivity. For example, doping germanium with arsenic or silicon with phosphorus results in n-type semiconductors.
Key characteristics of n-type semiconductors include:

• Excess of negative charge carriers (electrons)
• Higher electrical conductivity than intrinsic semiconductors
• Electrons are the majority carriers, while holes are the minority carriers

P-type Semiconductor

A p-type semiconductor is formed by doping a semiconductor material with an element that has fewer valence electrons than the host. Commonly, these impurity atoms belong to group III of the periodic table, such as boron, aluminum, and indium.
When these dopants are introduced, they create "holes" or vacant spaces where electrons can go. These holes act like positive charge carriers, enhancing the conductivity of the material. For instance, germanium doped with indium becomes a p-type semiconductor.
Important characteristics of p-type semiconductors include:

• Presence of positive charge carriers (holes)
• Better conductivity compared to undoped semiconductors
• Holes are the majority carriers, while electrons are the minority carriers

Germanium

Germanium is a chemical element with the symbol Ge and atomic number 32. It is a classic semiconductor material, sharing mechanical properties with silicon, though notably it has advantages in certain applications due to its higher electron mobility.
Germanium's typical four-valence electron structure allows it to form stable crystal lattices, easily accommodating the addition of impurities through doping.
Some interesting facts about germanium include:

• Found in the oxygen family of the periodic table
• Widely used in infrared optics, fiber optics, and solar cells
• Germanium does not occur naturally in significant quantities and must be extracted from minerals

Silicon

Silicon, symbolized as Si, is one of the most abundant elements on Earth and the cornerstone of modern electronics. Holding the atomic number 14, it forms the basis of most semiconducting materials used today, thanks to its excellent properties for electron conduction once doped.
Just like germanium, silicon forms a four-valent robust crystal lattice structure, allowing effective electron flow when appropriately doped.
Key aspects of silicon include:

• Belongs to the carbon group in the periodic table
• Highly used in computer chips, transistors, and other electronic devices
• Silicon's usage in semiconductor manufacturing due to its superior temperature stability compared to germanium

Silicon has largely driven the advancement of modern digital technology due to its effective semiconducting properties.